Ceramic membranes have a variety of industrial and scientific uses, the most common of which is use in separation processes. Organic membranes are often currently used in industry for separation processes, but ceramic membranes offer several advantages over organic membranes. Ceramic membranes are more resistant than organic membranes to organic solvents, chlorine, and, in some cases, extremes of pH. Ceramic membranes are also inherently more stable at high temperatures, thus allowing more efficient sterilization of process equipment than is possible with organic membranes. Ceramic membranes are generally quite resistant to microbial or biological degradation, which can occasionally be a problem with organic membranes. Ceramic membranes are also more mechanically stable under high pressures.
The mechanism of operation and types of separations which can be achieved by ceramic membranes are discussed in general by Asaeda et al., Jour. of Chem. Eng. of Japan, 19[1]: 72-77 (1986). At least one line of ceramic filters is currently marketed under the trade name "Ceraflo"by the Norton Company of Worcester, Mass. Alcoa Corporation of Pittsburgh, Pa. also markets ceramic filters made by slip-cast processing.
Ceramic membranes may be formed in particulate or polymeric manners. Anderson, et al., J. Memb. Sci 39: 243-258 (1988), describes different methods of making both particulate and polymeric sols from transition metal oxides. In general, particulate membranes have a smaller average pore diameter and a narrower pore size distribution as compared to polymeric membranes.
Particulate ceramic membranes are typically formed through a process beginning with organic-inorganic molecules. The molecules are formed into small metal oxide clusters which in turn aggregate to form metal oxide particles. The particles are fused into a unitary ceramic material. The gaps between the fused particles form a series of pores in the membrane.
The creation of these metal oxide ceramic membranes is generally conducted through a sol-gel procedure. Usually, the metal oxide is initiated into the process as a metal alkoxide solution. The metal is hydrolyzed to metal hydroxide monomers, clusters or particles, depending on the quantity of solvent used. The insoluble metal oxide particles are then peptized by the addition of an acid which causes the particles of the metal oxide to have a greater tendency to remain in suspension, presumably due to charges acquired by the particles during the peptizing process.
Such a sol can be evaporated to form a gel, which is a semi-solid material. Further evaporation, and then sintering, of the gel results in a durable rigid material which can either be formed as an unsupported membrane or as a supported membrane coated onto a substrate. This substrate can be either porous or non-porous and either metallic or non-metallic, depending on the particular application.
Two current limitations on the use of ceramic membranes are the materials used to fabricate the membranes and the membrane pore size and range. With regard to the composition of the membranes, ceramic membranes have been created using many materials. For example, Leenaars et al., Jour. of Membrane Science, 24: 261-270 (1985), report the use of the sol-gel procedure to prepare supported and unsupported alumina membranes. Ceramic membranes composed of titanium, zirconium and other metals have also been reported.
Silica represents an additional very advantageous element for use in ceramic membranes. Many workers have reported silica gels made by the hydrolysis and condensation of alkoxide starting materials. In previous methods of producing silica gels, acid-catalyzed hydrolysis lead to sols containing weakly-branched polymers and products having smaller pores while base-catalyzed hydrolysis lead to sols containing particles and products with larger pores. (Brinker, et al., Solids 48:47 (1982); Shafer, et al., J. Appl. Phys. 61:5438 (1987). Particulate silica gels have been prepared by hydrolysis of tetraethyl orthosilicate (TEOS) in a solution of water, ammonia and ethanol. The solution is usually stirred for a period of time until the silica particles form a colloidal sol. For example, Badley, et al., Langmuir 6: 792-801 (1990), have prepared monodispersive colloidal silica particles of 50-700 nm.
Ceramic membranes formed of transition metal oxides have been reported with pore sizes of 5 to 40 Angstroms. Anderson et al., U.S. Pat. No. 5,006,248 discloses aluminum ceramic membranes with small pore size. There has been one report of silica membranes with pore sizes under 50 Angstroms. Mary Gieselmann, University of Wisconsin, Water Chemistry Program, has synthesized silica gels and membranes by adapting the TEOS/ammonia/ethanol technique in light of Stober, et al., J. Colloid Interface Sci. 26:62 (1968), and Van Helden, et al., J. Colloid Interface Sci. 81:354 (1981). These silica membranes had an average pore diameter as low as 40 Angstroms (Gieselmann, personal communication).
Gieselmann's disclosure does not satisfy the need for a method of preparing micropore silica membranes. A disadvantage of Gieselmann's method is that the TEOS/ethanol/ammonia solution had to be stirred for one week before a sol formed. Also, the method relied on extreme dilutions to prevent particle aggregation, thus requiring large amounts of solvent. Additionally, there are many applications in which a membrane with pore size less than 20 Angstroms is needed. For example, for ultrafiltration, reverse osmosis, and gas separation.
What is needed in the art of ceramic membranes is an improved method of creating silica membranes with small pore size.